Q1 What does Jesus mean by: "If you have faith"?I think Jesus meant "if you have trust & confidence" in God. Alternatively, it could be viewed in light of Heb 11:1 "Now faith is the assurance of things hoped for, the conviction of things not seen". Hence without seeing the mulberry tree being uprooted in reality, if the apostles had the assurance that what they hoped for would be done... that would be Faith.
Q2 Who is a 'model' servant according to this passage? Why?I think the model servant is one who is humble & who recognises his true worth (unworthiness) before God. I think these small 4 verses serve to remind us that just because we may be doing some good or leading souls to Jesus etc, it should not go to our heads and make us feel like we are like God (to sit at His table).
Probably that's what happened to Lucifer, who must have been doing a good job with the music in heaven & thought he had the right to sit with God, forgetting his true place, as a created one, a servant of the most high God.
It's waaay so much more true for us, who have been forgiven of soo many sins, to realise our true unworthiness before God and keep ourselves humble & free from pride at all times.
Q3 What is the best reward for serving the Master?I agree with Seeker here. Serving itself is the best reward, and it is this service that will make us whole. A sort of 'good-circle' effect

Blessings,
Melody